The present invention relates to production of a metal by electrolysis of a metal compound in a cell having a cermet anode. More specifically, the invention relates to protection of the cermet anode and its support structure assembly from thermal shock during cell start-up.
A number of metals including aluminum, lead, magnesium, zinc, zirconium, titanium, and silicon can be produced by electrolytic processes. Each of these electrolytic processes employs an electrode in a highly corrosive environment.
One example of an electrolytic process for metal production is the well-known Hall-Heroult process producing aluminum in which alumina dissolved in a molten fluoride bath is electrolyzed at temperatures of about 960-1000xc2x0 C. As generally practiced today, the process relies upon carbon as an anode to reduce alumina to molten aluminum. The carbon electrode is oxidized to form primarily CO2, which is given off as a gas. Despite the common usage of carbon as an electrode material in practicing the process, there are a number of serious disadvantages to its use.
Because carbon is consumed in relatively large quantities in the process, approximately 420 to 550 kg carbon per ton of aluminum produced, the electrode must be constantly repositioned or replenished to maintain proper anode-cathode spacing to produce aluminum efficiently. If prebaked electrodes are used a relatively large facility is needed to produce sufficient electrodes to operate a smelter. In order to produce aluminum of sufficient purity to satisfy customer standards, the electrodes must be made of having relatively low metal content carbon, and availability and cost of raw materials to make the carbon are of increasing concern to aluminum producers.
Because of disadvantages inherent in use of carbon for electrodes, some cermet materials have been developed that can operate as electrodes with a reasonable degree of electrochemical efficiency and withstand the high temperatures and corrosive environment of the smelting cell. Cermet electrodes are inert non-consumable and dimensionally stable under cell operating conditions. Replacement of carbon anodes with inert anodes allows a highly productive cell design to be utilized, thereby reducing costs. Significant environmental benefits are achievable because inert electrodes produce essentially no CO2 or fluorocarbon or hydrocarbon emissions. Some examples of inert anode compositions are found in U.S. Pat. Nos. 4,374,050; 4,374,761; 4,339,088; 4,455,211; 4,582,585; 4,584,172; 4,460,905; 5,279,715; 5,794,112; 5,865,980; and 6,126,799, all assigned to Alcoa Inc. These patents are incorporated by reference.
Although cermet electrodes are capable of producing aluminum having an acceptably low impurity content, they are susceptible to cracking during cell start-up when subjected to temperature differentials on the order of about 900-1000xc2x0 C. In addition, ceramic components of the anode support structure assembly are also subject to damage from thermal shock during cell start-up and from corrosion during cell operation. Accordingly, there still remains a need for a means of protecting cermet electrodes and the anode support structure from thermal shock and corrosion.
A principal objective of the present invention is to provide a coating for protecting a cermet anode and its support structure assembly from thermal shock during cell start-up.
A related objective of the invention is to provide a coating for protecting the support structure assembly from corrosion during cell operation.
Additional objectives and advantages of our invention will be apparent to persons skilled in the art from the following detailed description of some preferred embodiments.
In accordance with the present invention there is provided an electrolytic cell for production of a metal by electrolytic reduction of a metal compound contained in a molten salt bath. Metals that may be produced electrolytically in accordance with the invention include aluminum, lead, magnesium, zinc, zirconium, and titanium.
A preferred embodiment of the invention relates to production of aluminum by electrolytic reduction of alumina dissolved in a molten bath containing aluminum fluoride and sodium fluoride. An electrolytic current is passed between a cermet anode and a cathode through the salt bath, producing aluminum at the cathode and oxygen at the anode.
The molten salt bath comprises aluminum fluoride and sodium fluoride, and may also contain calcium fluoride, magnesium fluoride, and/or lithium fluoride. The weight ratio of sodium fluoride to aluminum fluoride is preferably about 0.7 to 1.1. The bath ratio is preferably about 0.8 to 1.0 and more preferably about 0.96.
As used herein, the term xe2x80x9cinert anodexe2x80x9d refers to a substantially non-consumable anode having satisfactory resistance to corrosion and dimensional stability during the metal production process. At least part of the inert anode comprises a cermet material. As used herein, the term xe2x80x9ccermetxe2x80x9d refers to a material having a ceramic phase and a metal phase. Inert anodes of the present invention may be made entirely of a cermet material over a central metal core. When the cermet is provided as an outer coating its thickness is preferably about 0.1 to 50 mm, more preferably about 1 to 10 or 20 mm. The ceramic phase preferably makes up about 50-95 wt % of the cermet material, the metal phase about 5-50 wt %. More preferably, the ceramic phase comprises about 80-90 wt % of the cermet and the metal phase about 10-20 wt %.
The ceramic phase of the cermet can be composed of any suitable oxide material including one or more metal oxides selected from the group consisting of Ni, Fe, Zn, Co, Al, Cu, Ti, V, Cr, Zr, Nb, Ta, W, Mb, Hf, and any of the rare earth metal oxides and at least one additional oxide from the above list. A particularly preferred ceramic phase embodiment comprises iron, nickel, and zinc oxides.
The metal phase of the cermet material comprises a base metal, such as Cu and/or Ag replaced in whole or in part by, or mixed or alloyed with one or more metals selected from the group consisting of Co, Ni, Fe, Al, Sn, Nb, Ta, Cr, Mo, W, and the like. The metal phase also comprises a noble metal such as one or more metals selected from Ag, Pd, Pt, Au, Rh, Ru, Ir, and Os. A preferred metal phase comprises copper as the base metal with the addition of at least one noble metal selected from Ag, Pd, Pt, Au, and Rh.
The metal phase may be continuous or discontinuous. When the metal phase is continuous it forms an interconnected network or skeleton that increases electrical conductivity. When the metal phase is discontinuous, discrete particles of the metal are at least partially surrounded by the ceramic phase, which may increase corrosion resistance.
The types and amounts of base metal and noble metal contained in the metal phase are selected in order to reduce unwanted corrosion, dissolution, or reaction of the inert anodes, and to withstand the high temperatures to which the inert anodes are subjected during the electrolytic production process. For example, in the electrolytic production of aluminum, the production cell typically operates at a sustained smelting temperature above 800xc2x0 C., usually about 900-980xc2x0 C. Accordingly, the metal phase of inert anodes in such cells should have a melting point above 800xc2x0 C., more preferably above 900xc2x0 C., and optimally above about 1000xc2x0 C.
The metal phase typically comprises about 50-99.99 wt % base metal and about 0.01 to 50 wt % noble metal. Preferably, the metal phase comprises about 70-99.95 wt % of the base metal and about 0.05-30 wt % of the noble metal. More preferably, the metal phase comprises about 90-99.9 wt % base metal and about 0.1-10 wt % noble metal. For every numerical range or limit set forth herein, all numbers within the range or limit including every fraction or decimal between its stated minimum and maximum are considered to be designated and disclosed by this description.
Inert anodes useful in practicing the present invention are made by reacting a reaction mixture with a gaseous atmosphere at an elevated temperature. The reaction mixture comprises particles of metals and metal oxides blended together. In addition to the ceramic and metal portions, the mixture may also include an organic polymeric binder, plasticizer, and/or dispersant. These components are added in an amount of about 0.1 to 10 parts by weight per 100 parts by weight of the ceramic and metal particles. Some suitable binders include polyvinyl alcohol, acrylic polymers, polyglycols, polystyrene, polyacrylates, and mixtures and copolymers thereof. Preferably, about 0.3 to 6 parts by weight of the binder or other components are added to 100 parts by weight of the ceramic and metal mixture.
The blended ceramic and metal powders are sent to a press where they are isostatically pressed, for example at 5,000 to 40,000 psi, into anode shapes. A pressure of about 20,000 psi is particularly suitable for many applications.
The pressed shapes may then be sintered in a controlled atmosphere furnace supplied with an argon-oxygen gas mixture, a nitrogen-oxygen gas mixture, or other suitable gas mixtures. Sintering temperatures of between about 1,000 and 1,400xc2x0 C. are typically suitable. The furnace is typically operated at between about 1,350 and 1,385xc2x0 C. for two to four hours. If a polymeric binder is used, the sintering process will burn out any of the binder from the anode shapes.
The gas supplied during sintering preferably contains about 5-3,000 ppm oxygen, more preferably about 5-700 ppm oxygen and most preferably about 10-350 ppm oxygen. Lesser concentrations of oxygen can result in a product having a larger metal phase than desired, and excessive oxygen can result in a product having too much of a ceramic phase. The remainder of the gaseous atmosphere preferably comprises a gas such as argon that is inert to the metal at the reaction temperature.
Sintering anode compositions in an atmosphere of controlled oxygen content typically lowers the porosity to acceptable levels and avoids the bleed out of the metal phase. The atmosphere may be predominantly argon, with controlled oxygen contents in the range of 17-350 ppm. The anodes may be sintered in a tube furnace at 1,350xc2x0 C. for two hours. Anode compositions sintered under these conditions typically have less than 0.5% porosity when the compositions are sintered in argon containing between about 70 and 150 ppm oxygen.
The electrolytic cell of our invention includes a cermet electrode, a support structure assembly, and a chamber containing the molten salt bath. The cermet electrode is preferably an anode including a body having an outer surface portion. The support structure assembly includes a metal conductor electrically connected with the cermet anode. The support structure assembly may also include a part containing a high alumina ceramic material. As used herein, the term xe2x80x9chigh alumina ceramic materialxe2x80x9d refers to a ceramic material containing at least 80 wt % Al2O3, preferably at least 90 wt %, and more preferably about 95 wt %. The ceramic material may be a high temperature castable ceramic, a high temperature ceramic board, or a combination thereof.
The high alumina ceramic material comprises a refractory material, preferably a mixture of calcium aluminate cement and tabular alumina. Optionally, the refractory material may include about 0.5-5 wt % of zinc borosilicate frit. When desired, about 0.1 to 1 wt % boric acid may be added as corrosion retarder. Additional details of the high alumina ceramic material are found in La Bar U.S. Pat. No. 4,158,568, the disclosure of which is incorporated herein by reference.
A particularly preferred high alumina ceramic material contains a mixture of tabular alumina (xe2x88x9248 mesh Taylor series); calcium aluminate cement (sold by Alcoa World Chemicals under the trademark C-25); and boric acid. A suitable high alumina ceramic material sold by Permatech Inc. of Graham, N.C. under the trademark xe2x80x9cPermatech Alphaxe2x80x9d. Contains about 95 wt % Al2O3; about 4.0 wt % CaO; about 1.0 wt % other ingredients. The material has a bulk density of 180 lb/ft3 (2885 kg/m3), 18-20% apparent porosity, and 3.5 specific gravity. Maximum use temperature is 2600xc2x0 F. (1427xc2x0 C.).
An alternative high alumina ceramic material is sold by Harbision-Walker Refractories Co. under trade name SHAC (special high alumina castable). The material""s nominal composition is 94.8 wt % alumina, 4.9 wt % lime (CaO), 0.1 wt % MgO, 0.1 wt % alkalis, and trace silica.
In order to protect the cell from thermal shock during start-up, at least one of the outer surface portion of an anode and a portion of the support structure assembly is coated with a layer comprising carbon or aluminum or a mixture of carbon and aluminum A carbon layer is preferred. The carbon layer may have a thickness of about 0.1-10 mm. A particularly preferred carbon layer has a thickness of about 1-3 mm. The carbon layer is preferably applied by dipping or spraying.
A particularly preferred protective coating includes an underlayer comprising aluminum adjacent the outer surface portion of the anode, and a carbon overlayer coated over the underlayer. The aluminum underlayer may have a thickness of about 0.1-5 mm. A particularly preferred underlayer has a thickness of about 0.2-1 mm. The aluminum underlayer may be applied by a variety of coating methods including thermal spraying, electrocoating, electroless plating, physical vapor deposition, powder cementation, chemical vapor deposition, immersion, painting, and electrostatic spraying.